1. Field of the Invention
This invention pertains to a method of in situ remediation of contaminated groundwater. More particularly, this invention pertains to a method of in situ remediation of contaminated groundwater by stimulating the bio-degradation of contaminants by sulfate-reducing microorganisms.
2. Description of the Prior Art
The severity and extent of groundwater contamination are well known environmental challenges. One source of such contamination is hydrocarbon contamination of groundwater from petroleum-based spills or leakages. For example, underground storage tanks, above ground storage tanks, pipelines and the like may leak petroleum-based contaminants into the soil and associated groundwater. Similarly, surface spills of such contaminants can migrate into groundwater.
There are many suggested methods and techniques for treating contaminated soil or groundwater. Examples of these are found in U.S. Pat. Nos. 6,830,695; 6,805,518; 6,502,633; 6,497,534; 6,474,908; 6,457,905; 6,336,772; 6,276,871; 5,833,855; 5,759,402 and 5,525,008.
Recently, much attention has focused on in situ remediation by anaerobic (oxygen-lean or oxygen-free) bioremediation of contaminants by sulfate-reducing microorganisms. Such microorganisms are described in U.S. Pat. No. 5,833,855.
Only recently has scientific literature reported on exploiting in situ sulfate-reducing microorganisms to treat hydrocarbon-contaminated groundwater. For example, Anderson et al., “Anaerobic Bioremediation of Benzene under Sulfate-Reducing Conditions in a Petroleum-Contaminated Aquifer”, Environmental Science and Technology, Vol. 34, No. 11, pages 2261–2266 (2000) purports to be the first field study demonstrating stimulation of anaerobic benzene degradation in a petroleum-contaminated aquifer. That study suggests the addition of sulfate stimulated the activity of benzene-degrading, sulfate-reducing microorganisms.
One study suggests there may be only a few petroleum-related compounds that anaerobic microbial communities cannot degrade. The study states the capacity for self-purification of hydrocarbon-contaminated sediments may be greater than previously recognized. Coates, et al., “Oxidation of Polycyclic Aromatic Hydrocarbons under Sulfate-Reducing Conditions”, Applied and Environmental Microbiology, Vol. 62, No. 3, pages 1099–1191 (1996). In 2001, microbial mineralization of MTBE (methyl tert-butyl ether) was first reported under sulfate-reducing conditions. Bradley et al., “Effect of Redox Conditions on MTBE Biodegradation in Surface Water Sediments”, Environmental Science and Technology, Vol. 35, No. 23, pages 4643–4647 (2001).
While the application of sulfate to contaminated groundwater is promising and has received considerable scientific consideration, Applicant has concluded the effective use of this technology has been burdened by erroneous assumptions limiting the effective use of sulfate application to contaminated groundwater. Specifically, the environmental industry, scientific community and relevant government authorities have accepted as dogma that sulfate solutions as applied to contaminated sites should be limited to a sulfate concentration of not more than 250 parts per million (ppm).
The applied concentration limitation limits the efficacy and cost efficiency of this treatment. This limitation requires an enormous amount of solution be prepared and applied in order to treat a contaminated site. The enormity of the required solution drastically impacts the economic feasibility of sulfate application for in situ bioremediation of contaminated groundwater, and the technical feasibility of applying the solution into the subsurface through the use of wells. The enormous amounts of solution must be prepared, transported, and applied over a long period of time. Each of these processes significantly adds to the cost of in situ remediation.
Applicant has concluded that the recognized application concentration limits are unnecessary constraints on the effective application of sulfate to contaminated groundwater sites.
A summary for the basis of the concentration limitation can be found in the scientific literature. See, e.g., Cunningham, et al. “Enhanced In Situ Bioremediation of BTEX-Contaminated Groundwater by Combined Injection of Nitrate and Sulfate”, Environmental Science and Technology, Vol. 35, No. 8, pages 1663–1670 (2001). The Cunningham, et al., paper explains the rationale for the 250 ppm sulfate concentration limitation as well as describing techniques for applying sulfate solutions to contaminated groundwater through wells and techniques for monitoring the effects of a treatment.
The Cunningham, et al., paper is measuring BTEX concentrations in contaminated groundwater. BTEX is an acronym for gasoline constituents (i.e., benzene, toluene, ethyl-benzene and xylene) found as contaminants in groundwater.
Table 1 of the Cunningham, et al., paper (p. 1664) notes that sulfate (SO42−) is applied in an aqueous solution having a sulfate concentration not in excess of 250 ppm. Two reasons are given for the concentration limitation. First, the authors cite scientific literature that toluene degradation is inhibited if sulfide is produced. Second, the authors cite the United States government's secondary drinking water standard for maximum tolerable sulfate concentration for drinking water at 250 ppm. While water is not toxic at this concentration, the taste of the water can be adversely affected.
These factors have affected the environmental industry to limit sulfate applications to use sulfate solutions applied at a concentration of 250 ppm. Further, these constraints have been adopted by regulatory bodies within the United States.
The application of these constraints have significantly and adversely affected the beneficial effects of sulfate treatment programs. In order to be effective, an enormous volume of sulfate solution must be applied. Such solution can be applied directly on the surface of the contaminated site, through preexisting or specially drilled wells or in percolating tubes laid out in a grid or other pattern in trenches excavated at the site.
When a large volume of sulfate solution is required, the cost of preparing, transporting, and applying the sulfate solution can be prohibitive. Such a large volume of solution requires a very long application period. A rapid application of such a large volume of solution can result in surface runoff or it can flush contaminants out of the plume and simply cause the contaminants to migrate to another location without degrading the contaminants.
Applicant believes the scientific rationale for the constraints of 250 ppm is flawed notwithstanding the respected credentials of the advocates of such constraints. For example, even though the United States government and the various state regulatory bodies have adopted the 250 ppm standard to avoid unpleasant taste in drinking water, such standards are believed by Applicant to ignore that the sulfates do in fact break down in nature and the beneficial action of the sulfate-reducing microorganisms in contaminated soils and groundwater prevent the migration of sulfates into drinking water.
Further, it is Applicant's belief that, notwithstanding the concerns expressed in Cunningham, et al., supra, excess sulfide production and inhibition of toluene degradation do not occur in situ. Excess sulfide production has occurred in laboratory conditions but not significantly in situ where dissolved iron is present in the groundwater. In the absence of such dissolved iron, the sulfide combines with hydrogen to form hydrogen sulfide, which has an obnoxious odor. But with dissolved iron in the groundwater (as is typical), the sulfide combines with the dissolved iron as a harmless precipitate.
It is an object of the present invention to provide a method for treating contaminated groundwater with sulfates in a manner uninhibited by the doctrines and dogma of the prior art.